1. Field of the Invention
The present invention relates to a lubricating oil composition for diesel engines provided with an exhaust gas recirculation (EGR) system, a diesel engine provided with an EGR system comprising a lubricating oil composition, a method of lubricating a diesel engine provided with an EGR system and a method of screening a lubricating oil composition for effectiveness in lubricating a diesel engine provided with an EGR system. More particularly, the present invention relates to compression ignited internal combustion engines equipped with EGR systems in which intake air and/or exhaust gas recirculation streams are cooled below the dew point during operation (condensation mode), lubricated with a lubricating oil composition that provides acceptable performance over time in such an engine.
2. Discussion of the Background
Environmental concerns have led to continued efforts to reduce NOx emissions of compression ignited (diesel) internal combustion engines. The latest technology being used to reduce the NOx emissions of heavy duty diesel engines is known as exhaust gas recirculation or EGR. EGR reduces NOx emissions by introducing non-combustible components (exhaust gas) into the incoming air-fuel charge introduced into the engine combustion chamber. This reduces peak flame temperature and NOx generation. In addition to the simple dilution effect of the EGR, an even greater reduction in NOx emission is achieved by cooling the exhaust gas before it is returned to the engine. The cooler intake charge allows for better filling of the cylinder, and thus, improved power generation. In addition, because the EGR components have higher specific heat values than the incoming air and fuel mixture, the EGR gas further cools the combustion mixture leading to greater power generation and better fuel economy at a fixed NOx generation level.
Diesel fuel contains sulfur. In the United States today, even low sulfur diesel fuel may contain as much as 500 ppm sulfur, whereas European diesel fuel generally contains amounts of the order of 50 ppm. When fuel is burned in the engine, sulfur is converted to SOx. In addition, one of the major by-products of the combustion of a hydrocarbon fuel is water vapor. Therefore, the exhaust stream contains some level of NOx, SOx and water vapor. In the past, the presence of these substances has not been problematic because the exhaust gases remained extremely hot, and these components were exhausted in a disassociated, gaseous state. However, when the engine is equipped with an EGR, and the EGR stream is cooled before it is returned to the engine, the NOx, SOx, water vapor mixture is cooled below the dew point, causing the water vapor to condense. This water reacts with the NOx and SOx components to form a mist of nitric and sulfuric acids in the EGR stream.
In the presence of these acids, it has been found that soot levels in lubricating oil compositions can build rapidly, and that under such conditions, the kinematic viscosity (kv) of lubricating oil compositions increases more rapidly to unacceptable levels, even in the presence of relatively small levels of soot (e.g., 3 wt. % soot). Because an increased lubricant viscosity can adversely affect performance, and can even cause engine failure, the use of an EGR system that operates in a condensing mode during at least a portion of the operating time, requires frequent lubricant replacement. API-CI-4 oils developed specifically for EGR equipped engines that operate in a condensing mode have been found to be unable to address this problem. It has also been found that simply adding additional conventional dispersant is ineffective in reducing this viscosity growth.
Therefore, it would be advantageous to identify lubricating oil compositions that better perform in diesel engines equipped with EGR systems. Surprisingly, it has been found that by selecting certain additives, specifically certain viscosity modifiers and/or detergents, the rapid increase in lubricant viscosity associated with the use of engines provided with EGR systems that operate in a condensing mode can be ameliorated.
Traditionally heretofore, in addition to normal dispersant components, polymeric boosters such as N— dispersants VI improvers (e.g. Viscoplex series 6 polymers) have been added. (e.g. U.S. Pat. Nos. 4,290,925, 3,142,664, SAE 2003-01-1959, SAE 2002-01-1671, SAE 2000-01-1988) The reasoning for adding such N-dispersants was that oxidation occurs in the diesel engine which also creates acidic components. However, such acidic components are of a different chemical nature and quantity than those produced by a cooled EGR engines.
Ritchie et al. U.S. Pat. No. 6,715,473 describes an EGR equipped diesel engine and lubricating oil composition lubricating same.
Ritchie et al. US 2004/0485753 describes a lubricating oil composition containing less than 0.3% sulfur and comprising (a) a major amount of oil of lubrication viscosity, (b) an amount of nitrogen containing dispersant contributing no more than about 3.5 mmols of nitrogen per 100 grams of oil, wherein greater than 50 wt. % of the total amount of dispersant nitrogen is nonbasic and (c) one or more detergents, wherein about 60% to 100% of the total amount of the detergent surfactant is phenate and/or salicylate.
Seebauer et al. U.S. Pat. Nos. 6,124,249 and 6,271,184 describe viscosity improvers for lubricating oil compositions comprising:                a) C13-19 polyalkyl(meth)acrylates (PAMA);        b) C7-12 PAMA (branched with 2-C1-4 groups), and        c) optionally c1) C2-8 PAMA or c2) vinyl aromatic compounds and nitrogen containing vinyl monomers with <60% of the ester groups containing not more than 11 carbon atoms.The use of the polymer is in a gear oil formulation, mainly useful for continuous variable transmission fluids.        
U.S. Pat. No. 5,571,950 describes a method for testing for soot-related viscosity increase, comprising: (1a) obtaining a sample which comprises a major amount of an oil of lubricating viscosity; (1b) measuring the viscosity of the oil; (1c) preparing a stable sample/paste dispersion of the sample and carbon black paste; (1d) equilibrating the sample/paste dispersion; and (1e) measuring the viscosity of the sample paste dispersion, wherein shear is added to mimic the shear effects of an engine environment at any time after said step (a).
EP 937769 describes a copolymer comprising units derived from (a) methacrylic acid esters containing from about 9 to about 25 carbon atoms in the ester group and (b) methacrylic acid esters containing from 7 to about 12 carbon atoms in the ester group, said ester groups having 2-(C1-4 alkyl)- substituents, and optionally (c) at least one monomer selected from the group consisting of methacrylic acid esters containing from 2 to about 8 carbon atoms in the ester group atoms and which are different from methacrylic acid esters (a) and (b), vinyl aromatic compounds, and nitrogen-containing vinyl monomers with the proviso that no more than 60% by weight of the esters contain not more than 11 carbon atoms in the ester group. Also described are additive concentrates and lubricating oil compositions containing the copolymers and processes for preparing copolymers.
EP 0,750,031 describes a copolymer comprising:                a) 5-75 wt % C1-11 PAMAs;        b) 25-95 wt % C12-24 PAMAs; and        c) 0.1-20 wt % N-dispersants.        
U.S. Pat. No. 6,323,164 B1 describes a) 12-18% C1 PAMA; b) 75-85% C10-15 PAMA; and c) 2-5% N-dispersant monomers.
U.S. Pat. No. 4,867,894 describes a statistical PAMA-copolymer of Mw 50,000-500,000 with a) 10-30 mol % C1-PAMA; b) 10-70 mol % C16-30 PAMA; c) 10-80 mol % C4-15 PAMA; and d) 0-30 mol % oxygen or nitrogen dispersant as pour point depressants.
U.S. Pat. No. 4,968,444 describes a binary combination of statistical PAMA-copolymers I/II with I) 10-98 mol % C6-15; Ib) 0-5 mol % C16-30 PAMA; c) 0-90 mol % C8-40 PAMA; Id) 0-50 mol % C1-5 PAMA; and Ie) 2-20% oxygen or nitrogen dispersant PAMA; and
II) IIa) 0-90 mol % C6-15; IIb) 10-70 mol % C16-30 PAMA; IIc) 0-90 mol % C8-40 PAMA; IId) 0-50 mol % C1-5 PAMA; and IIe) 0-20% oxygen or nitrogen dispersant PAMA.
EP 439,254 describes an oil-soluble polymer, which comprises, as polymerized monomers, monomers selected from (a) alkylmethacrylates in which the alkyl group contains from 1 to 4 carbon atoms; (b) alkyl methacrylates in which the alkyl group contains from 10-15 carbon atoms; (c) alkyl methacrylates in which the alkyl group contains from 16 to 20 carbon atoms; and (d) N,N-dialkylaminoalkyl methacrylamides; and wherein said polymer contains:
i) 0-5% of (a); 74-97% of (b); ≦15% of (c); and 2-6% of (d) or
ii) <15% (a); 79-97% (b); and 2-6% (d).
The target is a fluid for internal combustion engines or automatic transmission fluids.
U.S. Pat. No. 4,021,357 describes a) 15-25% C1-5 PAMA; b) 62-40% C10-15 PAMA; c) 20-25% C16-20 PAMA; and (d) 3-10% N,N-dialkyl-aminoalkyl-methacrylamide. The target is fluid for internal combustion engines or automatic transmission fluids.
U.S. Pat. No. 5,756,433 describes a comb polymer, made via the macromonomer route from a) 0-90 mol % C6-30 PAMA; b) 0-60% C1-5 PAMA or Styrene or C1-4 alyklstyrene or C2-12 vinylesters of Carboxylic acids; and d) a “dispersion effective” amount of dispersant comonomers.
U.S. Pat. No. 5,843,874 describes a gear oil formulation comprising 0.1-10 wt % of polymer, said polymer consisting of a) 0-50 Wt. % C1-6 PAMA; b) 30-85 wt. % C7-14 PAMA; c) 3-50 wt. % of C15-20 PAMA; and d) 2-10 wt. % N,N-diaminoalkyl(meth)acrylamide.
U.S. Pat. No. 5,622,924 describes C1-C10 PAMA >70%, preferred 2-Ethyl-hexyl-methacrylate; C11-20 PAMA <30%, or other monomers.
EP 228,922 describes styrene (vinylaromatic Monomer) 10-35%; AMA=65-90%; C1-4 PAMA 5-15%; C8-14 PAMA 20-55%; C16-22 PAMA 15-50%.
U.S. Pat. No. 4,136,047 describes a lauryl-methacrylate=70-90%, styrene=10-30% composition.
U.S. Pat. No. 5,851,967 describes a graft copolymer, comprising:
92-28% of a polymer backbone, derived from
65-95 wt % C1-24 alkyl(meth)acrylate;
5-35 wt % styrenic monomer; and
2-8% branches grafted onto the backbone derived from (exclusively) C2-8 hydroxyalkyl(meth)acrylate.
U.S. Pat. No. 6,228,819 describes a method of making a viscosity index improver, comprising a) 5-70% C16-24 alkyl(meth)acrylate (AMA); b) 5-85% C7-15 AMA; c) 5-50% styrene; and d) 2-20% of a mix of (a) and (b).
JP 84020715 describes a polymer comprising:
a) 40-75% polymer 1, deriving from                0-80% C8-15 AMA;        20-100% C16-28 AMA; and        
b) 25-60% styrene.
The traditional polymeric N-dispersant boosters are not entirely effective in diesel engines equipped with EGR systems. Using traditional polymeric N-dispersant boosters, increases in oil viscosity are observed at lower soot concentrations, necessitating more frequent oil changes. It was therefore desirable to develop new boosters that can handle the soot and acidic compounds which are present in higher in amounts and are different in chemical nature.